Methanol three-column distillation

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Optimal Control of the Quality of Methanol Three-column Distillation Volume 9 Issue 2 2002 Author of Chemical Production and Technology: Zhao Shaomin Wang Lei Shao Lihong (Yankuang Lunan Fertilizer Factory, Tengzhou, Shandong 277527) Abstract: This article introduces the reasons why three-column distillation affects the quality of methanol products and the treatment methods, as well as the optimized operation and main technical transformation of methanol distillation. keywords: Methanol distillation quality technology improvement and control 1. Process flow introduction The 100 kt/a methanol unit of our factory adopts the most advanced low-pressure methanol production and energy-saving three-tower distillation process in China. The distillation system is a key link in controlling the quality of refined methanol. The quality of the distillation operation directly determines the quality of the product and the level of consumption. Since the start-up of the distillation unit in June 2000, we have promptly organized relevant personnel to carry out technical research in response to process and equipment problems such as large product quality fluctuations after the distillation unit was started, frequent negative pressure in the atmospheric tower and unstable pressure in the pre-distillation tower, and achieved obvious results. 2 Analysis of factors affecting the quality of methanol products Figure 1 Three-tower distillation process 1. Pre-distillation column ； 2 Pressurized distillation tower ； 3． Normal pressure distillation tower ； 4． Return tank ； 5 condenser ； 6． reboiler ； 7 Condensation reboiler ； 8 return pump ； 9 cooler ； 10. Preheater In the three-tower distillation process (see Figure 1), the main function of the preheater is to remove light component impurities such as methyl formate, dimethyl ether, and propylene. This type of substance has a low boiling point and is gaseous at room temperature. Therefore, the non-condensable temperature determines the removal effect of light components, which in turn affects the quality of refined methanol, that is, it affects the acidity and alkalinity of the methanol product, especially the harvesting date.: 2001-11-07 revision date: 2001-12-14 KMnO, value of methanol products Crude methanol contains heavy component impurities such as higher alcohols, olefins, alkanes and organic alcohols (also called fusel oil or isobutyl oil fraction). These substances have high boiling points. Some substances, such as ethanol, can easily form azeotropes with water. The boiling points of these azeotropes are close to the boiling point of methanol. As the methanol evaporates, the ethanol in the product exceeds the standard. The function of the pressurized tower and the normal pressure tower is to remove heavy components such as H 0 and ethanol to obtain high-quality methanol products. The coordinated operation of the pressurized tower and the atmospheric tower is the key to controlling water content. The normal pressure tower relies on the methanol vapor from the top of the pressurized tower to heat the liquid at the bottom of the tower. That is, the reboiler of the normal pressure tower is the condenser of the pressurized tower. Therefore, the temperature at the bottom of the pressurized tower determines the heat load of the normal pressure tower. This requires good control of the material balance, heat balance, and vapor-liquid balance of the two towers. Otherwise, it is easy to cause negative pressure in the atmospheric tower, causing the moisture content of the methanol product to increase. The problem of negative pressure in the atmospheric tower is the key to system stability and product quality. The reasons for the formation of negative pressure in the atmospheric tower must be analyzed from two aspects: process and equipment. 2.1 Equipment (1) The methanol steam pipe at the top of the atmospheric tower has leaks or is not insulated. It is especially prone to negative pressure in the atmospheric tower in winter. ； (2) The installation position of the pressure regulating valve of the atmospheric tower is unreasonable and the purpose of pressure regulation cannot be achieved. ； (3) The reboiler piping of the atmospheric tower is unreasonable, resulting in insufficient heat exchange area, resulting in too small heat load of the atmospheric tower. ； (4) The vent piping of the non-condensable gas in the pre-tower is not properly managed, causing the vent air from methanol synthesis to affect the pressure of the pre-tower, causing instability in the entire system. 2.2 In terms of technology, Zhao Shaomin and others optimized and controlled the quality of methanol three-column distillation technology and improved it.: (1) The heat load at the bottom of the pressurized tower is insufficient, resulting in insufficient heat load in the atmospheric tower, that is, poor heat balance control ； (2) The extraction volume of the pressurizing tower is too large and the reflux flow is too small, resulting in low methanol content in the bottom component of the pressurizing tower, that is, poor material balance control ； "3) The production volume of the atmospheric tower is too large and the reflux volume is too small, resulting in the destruction of the vapor-liquid balance on the trays of the atmospheric tower and the formation of negative pressure. The sign that the vapor-liquid balance of the atmospheric tower is destroyed is the rapid rise of several sensitive temperature points in the atmospheric tower. The 4th, 6th, 8th, and 12th trays rise to above 100°C (one of the feed plates of the atmospheric tower is on the 10th tray). This indicates that the heavy components begin to move upward. Once the time is too long and the treatment is not timely, the refined methanol extracted from the top of the tower is often unqualified. 3 Treatment measures and optimized operations 3.1 Measures taken in terms of equipment (1) The piping of the reboiler of the atmospheric tower was not managed properly, causing the methanol at the outlet of the reboiler to form a 0.5 m liquid seal, which reduced the heat exchange area of ​​the reboiler. In November 2000, the rectification was completed, the outlet liquid seal was eliminated, the heat exchange area was increased, and the bottom temperature of the atmospheric tower increased by 3 to 5°C. (2) The adjustment width of the normal pressure tower was moved from the first floor to the 7-meter platform on the second floor, and the liquid seal was canceled to achieve the purpose of pressure adjustment and achieve better results. (3) After the non-condensable gas pipeline in the pre-tower was changed to a discharge tank, the problem of unstable pressure in the pre-tower was solved, laying the foundation for the stable operation of the entire distillation system. 3.2 Measures taken in the process (1) Increase the methanol concentration at the bottom of the pressurized tower, increase the reflux volume of the pressurized tower, and reduce the production volume of the pressurized tower so that there is "something" to be mined in the atmospheric tower (this is one of the main reasons for negative pressure in the atmospheric tower). This requires that the production distribution of the pressurized tower and the normal pressure tower should be controlled reasonably, basically according to 4: 6 principles to extract refined methanol. (2) Increase the temperature of the reflux liquid in the atmospheric tower from the designed 4o°C to 50°C, and use the sensible heat of the reflux liquid as much as possible to speed up the evaporation of the methanol liquid (from this point of view, the outlet pipe at the top of the atmospheric tower should be insulated), thereby temporarily achieving the purpose of increasing the pressure at the top of the atmospheric tower. (3) Increase the bottom temperature of the atmospheric tower by increasing the bottom temperature of the pressurized tower to ensure the heat required for rectification of the atmospheric tower and solve the problem of insufficient heat load of the atmospheric tower. (4) To stabilize the pressure of the atmospheric tower, the operator should promptly adjust the reflux flow according to the temperature of the sensitive point of the fourth tray of the atmospheric tower, so that the temperature should be controlled to ≤9o. c. Reduce the production volume of the atmospheric tower and increase the reflux volume appropriately to ensure the vapor-liquid balance of the tower. (5) Based on operating practice, we set the non-condensable gas temperature from the designed